1. Field of the Invention
This invention relates to integrated circuits, and in particular, to apparatus for using integrated circuit memories in which one or more sections of the memory are defective, while one or more sections are functional.
2. Description of the Prior Art
One goal in developing integrated circuits is reducing the cost of components integrated on a chip. For a given process as more components are integrated onto a given chip, the chip size increases, yield decreases, and cost increases.
Phenomena which reduce the fabrication yield of integrated circuits may be broadly divided into two classes: performance failures due to changes in process parameters, and structural failures due to isolated defects. Performance failures generally ruin an entire chip, or wafer, while structural failures often yield a partially functional chip, particularly with memories, or other circuits including large numbers of identical components. For example, a structural detect in a 64k bit memory may ruin only a portion of the memory yet permit a "partial" product of 32k size to be produced.
Although potentially offering products in effect made from what would otherwise be discarded, the manufacture and sale of partial products has not been widespread because of a number of problems. The manufacturer of such devices must stock and sell different partial products as distinct products requiring different pin connections, thereby increasing overhead. The user of such devices may find it necessary to wire various sockets for the same size partial product differently, and must maintain an inventory of each partial product. Furthermore, if the manufacturer of a partial product has a shortage of one particular partial product, it may be forced to substitute products which are entirely good to maintain delivery schedules, or it may be forced to generate more partial products of the particular kind sought by increasing production. If production is increased, however, other partial products will be fabricated which may not be in demand.